In imaging optical systems, parallel light flux areas are often arranged within the imaging optical system for various purposes. For example, in some imaging optical systems a parallel light flux area is provided at a position where an a focal optical system is to be inserted in order to change the focal distance of the imaging optical system. As another example, a parallel light flux area may be arranged within an imaging optical system in order that a beamsplitter, such as a beamsplitting prism, may be positioned in the parallel light flux area in order to introduce light to a finder system or to an optical ranging system for an auto focus system. A parallel light flux area is also useful for placing other optical elements. One such example is an interference filter wherein the transmittance of the filter is sensitive to the angle of the incident light.
Optical elements, including prisms and filters, that are to be inserted in parallel light flux areas generally have one or more planar surfaces that are substantially orthogonal to the optical axis and are for transmitting light. The use of parallel light flux areas with such an optical element, herein termed a xe2x80x9cplanar elementxe2x80x9d for purposes of clarity of description, causes a problem of ghost images. These ghost images are created by light that is transmitted by the planar element but is then reflected back to the planar element by optical elements or image detecting surfaces that receive the transmitted light. The light reflected back to the planar element may, in turn, be reflected by the planar element back through the optical elements to an image surface that also receives the originally transmitted light, thereby creating a ghost image. Additionally, light reflected by the planar element may be again reflected by optical elements or image sensor surfaces that follow the planar element, thereby creating additional ghost images. Especially when using a CCD image sensor, which has an image surface with a higher coefficient of reflection than that of film, substantial amounts of light may be reflected in the optical system, and this will degrade the quality of the final image.
FIG. 5 is a side view of a prior art optical system that will be used to explain the occurrence of ghost images. The imaging optical system of FIG. 5 includes lenses L1, L2, and L3, and a collimated light flux from the object side that is parallel to the optical axis X will be focused by the lens L3 to the position P on the image surface 1. In the parallel light flux area C between lens L2 and lens L3, a beamsplitter 2 is arranged as a planar element and the light flux is split by the beamsplitter 2 into a first image surface side light beam and a second image surface side light beam.
Ghost images are generated by light that is reflected by image surface 1 and then reflected by a planar surface 3 that is generally orthogonal to the optical axis in the parallel light flux area C back to the image surface 1. As shown in FIG. 5, the primary light beam going in the direction 11 strikes the image surface 1. A portion of the incident light is reflected in the direction 12 and is refracted by lens L3 in the direction 13. Then a portion of that light is reflected on the planar surface 3 back along the same route, as shown by the direction arrows 14 and 15. A portion of the light that is reflected by the image surface 1 may also be transmitted by the planar surface 3 and reflected by the planar surface 4 in the direction 14 so as to reach the image surface 1 again.
FIGS. 6(a) and 6(b) show the appearance of ghost images on the image surface 1 of the optical system of FIG. 5 for on-axis and off-axis image points, respectively, with reference to a horizontal axis H and vertical axis V when enough light to produce ghost images is present in the optical system. As shown in FIG. 6(a), the primary image Pc of the object is imaged in the central position of the image surface 1 and the ghost image Gc appears in the neighborhood of the center of the image detecting surface with a similar shape to that of the primary image. As shown in FIG. 6(b), when the primary image Pd is off-axis, that is, away from the center of the image surface 1, the ghost image Gd appears in a position that is symmetrical to the image Pd of the object with respect to the center point of the image detecting surface, and with a shape similar to the primary image.
The ghost image Gd of every off-axis image point is displaced from its proper primary image Pd on image surface 1. Because the various planar surfaces that are orthogonal to the optical axis reflect substantial amounts of light and that light is reflected back and forth through the optical system, the ghost images may be visible when one or more planar surfaces orthogonal to the optical axis are present in the parallel light flux area C. Thus, this light will reduce the quality of the final image unless measures are taken to reduce the amount of reflected light or prevent reflected light from reaching the image surface.
Adding an anti-reflection coating on the planar element is a known way of reducing the amount of light in the ghost images. Japanese Laid Open Application No. 2000-036917 discloses a method of reducing unwanted reflected light by applying an anti-reflection coating on a planar surface of a filter. However, because about 0.1-0.2% of the reflected light remains by this method, a substantial amount of unwanted reflected light is still imaged from bright light sources, such as the light from car headlights or from the sun, creating ghost images that are considered unacceptable for high performance video cameras and digital cameras. As a result, in an imaging optical system that requires a high performance in the prevention of ghost images, it has been considered desirable to avoid placing an optical element having a planar surface generally orthogonal to the optical axis in a light flux area where at least the central portion of the light flux area is generally parallel to the optical axis.
As another method of ghost image prevention, it is known to make the surfaces of optical elements that are generally orthogonal to the optical axis some shape other than planar. For example, Japanese Laid Open Application No. 2000-249820 discloses making a plate filter unit in a bent shape with curved surfaces. However, it is difficult and costly to fabricate this kind of filter.
The object of the present invention is to provide an imaging optical system having a simple and inexpensive construction that prevents ghost images, even in the case where the imaging optical system includes an optical element that has one or more planar surfaces that are generally orthogonal to the optical axis of the imaging optical system and which are located in a light flux area before the image surface, with at least the central portion of the light flux area having light rays that are generally parallel to the optical axis. The present invention is especially useful in imaging optical systems, such as video cameras and digital cameras, that use an image sensor having a planar image surface with a high coefficient of reflection.